The present invention relates to a heat treating apparatus including a heating apparatus and a cooling apparatus incorporated in a semiconductor manufacturing system for manufacturing a semiconductor device.
In a photolithography process for manufacturing a semiconductor device, a semiconductor wafer is repeatedly heated and cooled. In the apparatus disclosed in, for example, U.S. Pat. No. 5,664,254, a wafer is heated in an adhesion unit and a vapor of hexamethyl disilazane (HMDS) is allowed to act on the wafer surface. Also, the wafer is heated in a pre-baking unit for stabilizing a photoresist coating film. The wafer is also heated in a post-baking unit for baking (PEB) the photoresist film after light exposure in a predetermined pattern. On the other hand, the wafer heated in the pre-baking step and the PEB step is cooled in a cooling unit to about room temperature in a predetermined time.
In these heat treatments, it is important to control the temperatures of the wafer, the hot plate and the cooling plate. However, it is very difficult to control accurately the actual temperature of the wafer during the heat treatment. For example, it is proposed to measure the surface temperature of the wafer by using an infrared ray observing apparatus or a temperature sensor. However, any of these proposed methods is low in accuracy and, thus, has not yet been put to practical use.
Some of a large number of wafers are deformed and warped. If a warped wafer W is disposed on a hot plate, only a small portion of the wafer W is brought into contact with a hot plate 58, as shown in FIG. 11A, leading to a small contact area between the hot plate 58 and the wafer W. Since the heat exchange efficiency between the warped wafer W shown in FIG. 11A and the hot plate 58 is lower than that between the wafer W that is not warped as shown in FIG. 9A and the hot plate 58, the warped wafer W takes a longer time for the heating by the hot plate 58 to a predetermined temperature.
On the other hand, the temperature of the hot plate 58 is detected by a sensor buried in the hot plate 58. Therefore, if the amount of heat released from the hot plate 58 into the wafer W is small, the electric energy required for maintaining the hot plate 58 at a target temperature is also small, resulting in a short time for the power supply to the heater. Where the warping of the wafer is small, the fluctuation in the heat amount supplied to the hot plate 58, which is derived from the warping of the wafer W, can be compensated by the heat capacity owned by the hot plate 58 itself.
However, where the wafer W is warped greatly, the temperature of the hot plate 58 is excessively lowered, compared with the case where the wafer W is not warped. In such a case, it is possible for the actual temperature of the hot plate 58 not to be elevated to reach a target temperature before the succeeding wafer W is disposed on the hot plate after removal of the heat-treated wafer W from the hot plate 58. Incidentally, the treating time for one cycle of the heat-treating step in the photolithography process is said to be 90 seconds or less. If the succeeding wafer W is subjected to a heat treatment under the state that the actual temperature of the hot plate 58 is not elevated to a target temperature within a predetermined time, the wafer W is likely to become defective.
In the conventional heat treatment apparatus, the heater of the hot plate 58 is turned on or off depending on the temperature of the hot plate 58. To be more specific, the power source of the heater is turned on or off or the power supply amount is increased or decreased depending on the condition as to whether or not the temperature of the hot plate detected by the sensor has reached a predetermined temperature. Therefore, power is supplied uniformly to the hot plate 58 in the conventional apparatus regardless of the presence or absence of the wafer W on the hot plate 58.
However, the hot plate 58 having the wafer W disposed thereon has a large heat capacity, compared with the hot plate 58 having the wafer W not disposed thereon and, thus, exhibits a low rate of the temperature elevation, with the result that a long time is required for heating the hot plate 58 to a target temperature such that the heating time possibly exceeds the processing time (90 seconds or less) for one cycle of the heat treating step included in the photolithography process.
It should also be noted that, if the heating time to a target temperature is unduly long, the wafers W are rendered nonuniform in the time during which the wafers W are actually subjected to a heat treatment, even if the wafers W are kept disposed on the hot plate 50 for the same time. It follows that the treated wafers W are rendered nonuniform in properties.
For example, if the wafer W is disposed on the hot plate 58 at time t4 as shown in FIG. 10A, the heat energy is transferred from the hot plate 58 to the wafer W, resulting in a rapid temperature drop of the hot plate 58. As apparent from curve A shown in FIG. 10B, the temperature of the hot plate 58 at time t5 immediately after time t4 is made lower than an allowable lower limit T2 (.degree. C.) of a target temperature .theta.a. The heater power source is turned on at time t5 so as to start heating of the hot plate 58. However, it takes time for the temperature of the hot plate 58 to be elevated to fall within an allowable range T1 to T2(.degree. C.) of the target temperature .theta.a, as shown in FIG. 10C. Naturally, the time during which the wafer W is actually subjected to a heat treatment is shorter than the time during which the wafer W is disposed on the hot plate 58. Since it is very difficult to control the time during which the wafer W is actually subjected to a heat treatment, the treated wafers W differ from each other in the actual heat treating time, giving rise to a non-uniform heat treatment.
The actual temperature of the hot plate 58 is not necessarily equal to the temperature detected by a temperature sensor. For example, it is possible for the temperature detected by the temperature sensor to be 19.degree. C., though the actual temperature of the hot plate 58 is 20.degree. C. The difference between the actual temperature and the temperature detected by the temperature sensor is called herein offset. The offset is caused by a complex factor involving a deviation derived from the characteristics themselves of the temperature sensor and a deviation that is brought about when a temperature detection signal is transmitted through an analog circuit or an amplifier.
Where there is an offset, the analog circuit or the amplifier are electrically adjusted to set forth seemingly that there is no offset. For example, where the temperature sensor has detected a temperature of 19.degree. C., the actual temperature of the hot plate 58 is handled as 20.degree. C. so as to display 20.degree. C. on a monitor screen. Where the offset is constant, it is possible to electrically adjust the analog circuit or the amplifier to set forth seemingly that there is no offset even where there is an offset.
However, the offset value varies with time in some cases. Suppose, for example, that a coating unit installed in a user's factory is operated by way of trial to confirm that there is an offset of 1.degree. C. between the actual temperature (20.degree. C.) of the hot plate and the temperature (19.degree. C.) detected by the temperature sensor, and that the unit is adjusted to eliminate seemingly the offset. In this case, the offset value of 1.degree. C. is maintained in the initial period of operation of the coating unit such that 20.degree. C. is displayed on the monitor screen. However, a temperature of 19.degree. C. lower than the set temperature (20.degree. C.) of the hot plate comes to be displayed on the monitor screen 6 months to one year after the start-up of the coating unit operation. In this case, the actual temperature has been found to be 20.degree. C. when examined in detail by using a high precision temperature measuring apparatus.
The deviation of the displayed temperature from the actual temperature of the hot plate is caused by two factors. A first factor is an increased offset derived from deterioration with time of the temperature sensor. A second factor, which is irrelevant to the temperature sensor, is a change in the characteristics of the analog circuit or the amplifiers or a change in the characteristics of the heat circulation system of the hot plate. For eliminating the first factor, the temperature sensor must be renewed. The second factor can be dealt with by an electric adjustment. However, the unit operation must be stopped completely for finding the cause of the offset, leading to reduction in the working ratio of the unit.